1. Field of the Invention
The present invention is generally related to the fabrication of integrated electronic circuits by collision free, neutral beam etching and, more particularly, to a method of improving the reactivity between a substrate surface and incoming etchant molecules by delivering the etchant molecules as highly excited neutral molecules or clusters of molecules.
2. Description of the Prior Art
The use of plasma-assisted etching techniques for circuit fabrication has gained increasing popularity in the semiconductor industry during the past 20 years. The primary advantage of plasma-assisted etching over the prior art wet chemical etching techniques is the superior dimensional control. Typically, integrated circuits are fabricated by building up successive layers of materials on a silicon substrate including a top layer of a photoresist. A resist pattern is created by selective exposure of portions of the photoresist to ultraviolet light followed by development. Subsequently, the underlying layers not coated with the photoresist are then etched away by particle beams which can include ion atoms, molecules or some combination thereof.
Fundamental to the accurate replication of the photoresist pattern into the underlying layer are the concepts of anisotropy and selectivity. FIG. 1A shows a structure 7 comprised of substrate 1 coated with a thin insulating layer 2, a polysilicon layer 3 and a photoresist 5. A resist pattern 9 is formed on the polysilicon layer 3 leaving gap 10 through which polysilicon layer 3 can be etched. A selective isotropic etch of gap 10 with little or no etching of the initial resist pattern 9 yields an undercut profile 11. A selective anisotropic etch as shown in FIG. 1B, however, provides an excellent transfer of resist pattern 9 to the polysilicon layer 3 to yield profile 13. FIG. 1C shows the results of a nonselective anisotropic etch. Erosion of the photoresist areas 15 and underlying polysilicon areas 17 has occurred and the etch has extended into area 19 of the insulating layer 2. Conversely, a controlled slope may be prepared by utilizing a selective anisotropic etch as shown in FIG. 1D. The initial resist pattern 5 is etched to create a slope on edge 21, which is then extended along edge 23 when polysilicon layer 3 is etched.
Generally, gas etching processes require a trade-off between anisotropy, selectivity and etch rate. Anisotropy as produced by positive ion bombardment can be achieved by three mechanisms: ions can sputter adsorbed reaction products from the surface, ions can produce lattice damage thereby creating more active sites for reaction, or ion bombardment can remove an adsorbed layer that inhibits etching by reacting with the etchant or inhibiting access to the surface by the etchant. In all three mechanisms, the ions are directed perpendicular to the surface and result in an anisotropic etch.
In one prior art method known as reactive ion etching (RIE), an anisotropic etch is accomplished by immersing the surface to be treated in a plasma consisting of chemically reactive radicals and ions from a parent gas. RIE has the advantage of being easily interfaced to multichamber integrated processing tools designed to minimize contamination, but high energy ions and photons in the plasma can damage the silicon lattice or thin oxide layers and fluorine in the plasma can etch silicon as well as silicon dioxide. Moreover, because the relative amounts of ions and radicals cannot be independently controlled, it is necessary to operate at a low pressure and low concentration of reactive species, which substantially reduces the etch rate.
In a second well-known method known as ion beam assisted etching (IBAE), etch rate is improved by the addition of a reactive beam consisting of reactive molecules such as Cl.sub.2 or NF.sub.3 to an ion beam which is directed at the surface to be treated. However, to accommodate both beams, the ion beam is typically directed perpendicular to the wafer surface while the reactive beam is applied at an angle to the wafer surface. Because of its angle, the reactive beam causes the undesirable effects of under-etching and undercutting.
U.S. Pat. No. 4,734,158 to Gillis discloses a method which attempts to overcome the problems associated with IBAE by generating the ion beam separate from the radical beam. The radical beam is directed perpendicular to the wafer surface, while the ion beam is initially generated at an angle to the substrate surface but then deflected so that it strikes the substrate generally coaxially with the radical beam. However, this method is somewhat complex because it requires individual control of the substrate movement and beam intensities.
U.S. Pat. No. 4,937,094 to Doehler et al. discloses a method of generating a high flux of activated species from an energy transferring gas. The method includes introducing an energy transferring gas into an enclosure, which is maintained at a subatmospheric pressure, through at least one aperture formed in a conduit. The flow of gas through this conduit is increased to a substantially transonic velocity and radio frequency (RF) or microwave frequency energy is utilized to activate the gas. The resulting plume of activated species of the energy transferring gas is then directed toward a substrate surface. The result is either deposition or etching of the surface.